Problem: Christopher is 3 times as old as Luis and is also 8 years older than Luis. How old is Christopher?
Answer: We can use the given information to write down two equations that describe the ages of Christopher and Luis. Let Christopher's current age be $c$ and Luis's current age be $l$ $c = 3l$ $c = l + 8$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $c$ is to solve the second equation for $l$ and substitute that value into the first equation. Solving our second equation for $l$ , we get: $l = c - 8$ . Substituting this into our first equation, we get the equation: $c = 3$ $(c - 8)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $c = 3c - 24$ Solving for $c$ , we get: $2 c = 24$ $c = 12$.